In general, vehicle power transmission apparatus generally uses so-called multiple disk clutches to arbitrarily transmit or cut off power of an engine to or from the vehicle wheels. In such a power transmitting apparatus, it has an input member connected to an engine side, an output member connected to a driven side, and a clutch member connected to the output member. The power can be transmitted by pressure-contacting a plurality of driving clutch plates, mounted on the input member, with a plurality of driven clutch plates, mounted on the clutch member. The power can be cut off by releasing the pressure-contacting force between the driving and driven clutch plates.
The prior art (e.g. European Laid-open Patent Specification No. 1058018) discloses technology which provides cams to add a pressure-contact assist function to improve the pressure-contacting force of both the driving and driven clutch plates during power transmission. Also, they provide a back-torque limiting function to release the clutch plates of the input and output sides when the rotational speed of the output member has exceeded that of the input member. According to this power transmitting apparatus, two cams are formed by providing gradient on each engaging surface of a clutch housing and a driving clutch plate. Thus, both the driving and driven clutch plates are strongly pressure-contacted by one pair of cams (pressure-contact assist cams) during the normal power transmission. On the contrary, the driving clutch plates are moved by the other pair of cams (back-torque limiting cams) toward a direction where the pressure-contact between the driving and driven clutch plates is released when back-torque is present.
However in the power transmitting apparatus of the prior art, since the gradient angles of the two cams (pressure-contact assist cams and back-torque limiting cams) are the same to each other, the described below problem exists. In order to firmly avoid trouble (locking of wheel) due to back-torque, it is preferable to set the gradient angle of the back-torque limiting cams as large as possible. This separates the driving clutch plates and the driven clutch plates away from each other as much as possible. Meanwhile, if the gradient angle of the pressure-contact assist cams is set large, cam surfaces of the pressure-contact assist cams and back-torque limiting cams interfere with each other due to the “wedge effect”. Thus, the driving force cut-off operation cannot be carried out effectively. In addition, if the gradient angle of the pressure-contact assist cams is set too large, the bearing pressure between the driving and driven clutch plates would be remarkably different from that originally set at the designed time and thus the operability would also be reduced.